ROS2 Tutorial

ROS2 tutorial by GuyueHome
Version information:

ROS2 vs. ROS1

Different:

• ROS2: No need of rosmaster anymore. Using DDS communication mechanism.
• ROS2: More OOP-style programming.

Similar:

• Similar framework and communication concepts: topic, service, action etc.

Install ROS2

Check the official documentation
What if I have to install ROS2 with ROS already in my PC?

• Settings for ROS2 coexistence with ROS.

Test your ROS2

1) Talker and listener

Open two terminals, first source the ros2 to both of them, then

ros2 run demo_nodes_cpp talker

for a demo-publisher wrote with C++ on one terminal and

ros2 run demo_nodes_py listener

for a demo-subscriber wrote with Python on the other terminal.

2) Turtlesim

Open 2 terminals, first source the ros2, then

ros2 run turtlesim  turtlesim_node

to launch the turtlesim node and

ros2 run turtlesim turtle_teleop_key

to run the keyboard control node.

Common-used ROS2 commands

1) ros2 node + Enter: check all the available cmds for node

• ros2 node list check the active nodes;
• ros2 node info /node-name check the information of a certain node;

2) ros2 topic + Enter: check all the available cmds for topic

• ros2 topic list
• ros2 topic info /topic-name
• ros2 topic echo /topic-name print the message from a topic in real-time;
• ros2 topic pub --rate 1 /topic-name message-type "{title1: {x: val, y: val, z: val}, ...}" publish a message to the topic;
  eg. Publish a velocity command of type geometry_msgs/msg/Twist to turtlesim's topic /turtle1/cmd_vel:

ros2 topic pub --rate 1 /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}"

3) ros2 service + Enter: check all the available cmds for service

• ros2 service call /service-name service-type {service-parameters}
  eg. Add another turtle into the turtlesim

ros2 service call /spawn turtlesim/srv/Spawn "{x: 2, y: 2, theta: 0.2, name: ''}"

4) ros bag

• ros2 bag record /topic-name eg. record the /turtle1/cmd_vel

ros2 bag record /turtle1/cmd_vel

The recorded data would be saved to the current path of the terminal.

• ros2 bag play bag-name eg. play the recorded /turtle1/cmd_vel

ros2 bag play rosbag2_2022_10_03-12_49_13

Here replace the bag name with yours correspondingly.

Configure the ROS2 developing environment

0) rosdepc install dependencies for an existing ros project

rosdepc is a tool that can help to install all the dependencies for a workspace automatically.

install and initialize rosdepc

sudo pip3 install rosdepc
sudo rosdepc init & rosdepc update

install dependencies for workspace

# cd to the workspace root path
rosdepc install -i --from-path src --rosdistro galactic -y

1) Workspace

mkdir -p ~/colcon_ws/src  # create workspace and src folders
cd colcon_ws
colcon build  # compile the workspace

after colcon build the folder structure would be like

colcon_ws
├── build
│   └── COLCON_IGNORE
├── install
│   ├── COLCON_IGNORE
│   ├── local_setup.sh  # source install/local_setup.sh
│   ├── _local_setup_util_sh.py
│   └── setup.sh
├── log
│   └── ...
└── src  # contains the packages

2) Package

ros2 pkg create --build-type <build-type> <package_name>

create a package

cd ~/colcon_ws/src
ros2 pkg create --build-type ament_cmake learning_pkg_c       # C++
ros2 pkg create --build-type ament_python learning_pkg_python # Python

for c++ package, there should be package.xml and CMakeLists.txt.
for python package, there should be package.xml, setup.cfg and setup.py.
The information of dependencies for rosdepc is exactly provided by package.xml.

ROS node

What is node?

• execute a certain process of the App;
• an independent .exe file;
• compliable for multiple programming languages, e.g. cooperation of a talker.cpp and a listener.py;
• can be launched on differnent host machines distributedly.
• be managed with a sole node name.

0) Source the ROS2 to the current terminal before launching VSCode

Sometimes we might encounter problems like "Import rclpy cannot be resolved". This can happen when we didn't link to ROS2's shell environment correctly, especially we have ROS and ROS2 coexist and any of them is NOT sourced defaultly to our terminal.
Actually, the necessary libraries rclpy and rclcpp are supposed to be installed along with ros2.

• (If not, install the library rclpy manuelly: https://index.ros.org/r/rclpy/)

According to the official guide, we can avoid VS Code's resolving shell environment phase by launching VS Code directly from a fully initialized terminal:

source /opt/ros/galactic/setup.bash  # link to ROS2's shell environment
cd colcon_ws/src
code .

1) Write the node: Object-oriented Programming (OOP)

• create package
• create script
  • add this at the begining of the .py script

    #!/usr/bin/env python3
    # -*- coding: utf-8 -*-

• make executable .exe file
  • C++: configure CMakeList.txt
  • Python: via the command chmod +x node.py

2) Compile the node

Python

Configure the setup.py: add the node entry in entry_points:
e.g.:

entry_points={
        'console_scripts': [
        'node_helloworld = learning_node.node_helloworld:main',
        ...
        ],
    },

Note that every time we've modified the .py script we need to re-build the workspace so that the changes could be updated, which is different from ROS1.

• for ROS1, the Python node we would run is exactly the script we've modified in /src
• for ROS2, the Python node is in /install, generated from the script via $ colcon build.

3) Run the node

We MUST source the setup.sh before running a ros node, which is likely to what we did with ROS1

cd colcon_ws
source install/setup.sh  # for ROS1, it was $ source devel/setup.bash
ros2 run learning_node node_helloworld

Command syntax:

$ ros2 run <pkg_name> <node_name>

C++

<node_name> stands for an .exe file which was built by CMake during compiling;

Python

<node_name>, or exactly <node_name.py> is an .exe file directly converted from the .py script.